Following a decline in the polls, criticized debate performances and accusation of improper conduct with a number of women, the Republican presidential nominee has increasingly suggested that the election is "rigged" against him. Trump has also encouraged his supporters to monitor voting sites to be on the lookout for what he has referred to as large-scale fraud.

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But Kasich, who is not voting for Trump, said the businessman is creating mistrust in the election process that is harmful.

"I don't believe that we have any massive fraud," Kasich said. "It creates doubt in people's minds, and I'm worried about 25% of Americans who may say when the election is over, it was stolen."